Well the device has an avalanche rating - that's how much energy it can take in a single breakdown event...
To calculate the dissipation in the gate resistor I think the formula is f x C x V^2 where f is switching frequency,
C is the effective capacitance of the gate, V is the gate drive voltage. Or you can substitute total gate charge for
C x V, giving f x V x Q (if you think about it that's just I(average) x V). R is really the total resistive loss in the gate circuit
of course, but if you add a gate resistor it is usually dominant.